In multimodal polyethylene resins the high molecular weight fraction usually with comonomer incorporation is responsible for the strength, including long term strength and how well this fraction is incorporated in the total polymer mass is the key for the final product properties, in particular for high-strength and high performance products such as PE100 or PE100+ pipe resins, high-end film resins or blow moulding resins.
Multimodal polyolefins, especially multimodal polyethylenes are inherently difficult to homogenize due to a large difference in viscosities and a large difference in particle size of the various reactor powder particles. Especially in sequential polymerization processes the high molecular weight and high viscous powder particles are normally considerably larger than the lower molecular weight particles. During the homogenization step the particles of high molecular weight polyethylene are difficult to homogenize within the polymer melt so that so-called “white spots” occur in the compounded material. These white spots usually have a size of below 10 to about 50 micrometer, even though they occasionally can have a size of up to 0.1 mm or even 0.5 mm, and consist of high molecular weight polymer particles that have not been adequately dispersed in the composition. Further, when compounding polymer compositions e.g. for the production of films gel particles with a size of about 0.01 to 1 mm often occur. These gel particles also consist of high molecular weight polymer particles not adequately dispersed and appear as disfiguring inhomogeneities in the finished film. Still further, inhomogeneities in multimodal polymer compositions may also increase roughness of the surface of articles produced thereof.
One possibility to break up these high viscous particles is to use higher shear forces during compounding. Higher shear forces are usually applied to the extent which is necessary with regard to the needs, the degradation limits of the polymer, energy costs, costs for necessary process stabilizers and other physical limits such as low viscosities and lower shear forces due to high temperatures generated and limited cooling capacity.
High shear forces, however, applied as shear flow which is the predominant flow in extruders and mixers, are in most cases not sufficient to break up high molecular weight polymer particles in multimodal polyethylene resins with large viscosity differences between the polymer fractions.
These compatibility problems particularly apply in the case an ultra high molecular weight fraction (UHMW) is included into a polyethylene composition for further improving strength properties, as it becomes more and more difficult to homogenize the ultra high molecular weight particles into the polymer matrix.
Thus, there is still a need for methods to incorporate ultra high molecular weight fractions into multimodal polyethylene resins as such that a homogeneous blend with a minimum of UHMW particles, so-called white spots is obtained at a minimum degradation of the polymer chains.
It has surprisingly been found that this object can be achieved when polymerizing the UHMW polyethylene fraction together with a polyethylene resin of lower weight average molecular weight in a two steps of a sequential multistage polymerization process to form a so-called in-situ master batch. This in-situ master batch is subsequently blended with a polyethylene resin to form the desired polyethylene composition. Said composition surprisingly shows a low amount of white spots even when blended under mild conditions in order to avoid degradation.